Blender 2.74 exporting custom normals without applying split normals modifier
As @Jerryno said, from python you can only get modifiers’ results by applying them into a temp mesh (using Object.to_mesh()), there is no reason for 'EditNormals' modifier to behave differently.

You can also have a look at some official exporters supporting 'apply modifier' feature (like e.g. .obj exporter), that’s what they are doing.